1. Field of the Invention
The present invention relates to heart stimulating devices and systems. In particular to such devices and systems that have the ability to detect evoked responses to stimulation pulses delivered to the heart of a patient. The invention also concerns a method of determining a capture verification condition.
2. Description of the Prior Art
Several different devices for stimulating a heart are known. Such devices are able to deliver stimulation pulses to one or more of the different heart chambers: the left ventricle, the left atrium, the right ventricle and the right atrium. The devices can often be implanted in a patient. The devices are normally also able to sense the electrical activity of the heart.
In connection with such devices, it is known to detect the capture of the heart, i.e. to detect whether the heart actually reacts as intended to a delivered stimulation pulse. If the heart is not captured (i.e. loss of capture) it is possible to arrange the device to deliver a back-up pulse with a higher pulse energy than the first pulse. It is also possible to increase the pulse energy in future stimulation pulses if capture is not detected. In order to save battery it is important that the stimulation pulses are not delivered with an unnecessarily high energy. By varying the energy of the stimulation pulses and by detecting the capture it is possible to find a threshold value for the stimulation pulse energy. Based on the threshold value, a suitable stimulation pulse energy can be determined.
The detection of capture, i.e. the detection of an evoked response (ER), can be done in different manners. Normally an IEGM (intracardiac electrogram) signal is detected within a time window (ER window) following a delivered stimulation pulse. The determination whether the detected signal indicates a capture can be performed in different manners. It is for example known to use the maximum amplitude of the detected signal within the ER window. It is also known to use a slope or derivative (usually the maximum slope) of the detected signal within the ER window. A third known possibility is to detect an area by integrating the detected signal in the ER window.
The detection of capture involves different problems. One problem is the electrode-polarisation. The electrode-polarisation is a residual voltage that appears at the electrode used for the stimulation. In particular if the same electrode is used for emitting the stimulation pulse and for sensing the evoked response, the electrode-polarisation can make the detection difficult.
It is also known that the delivery of stimulation pulses and the detection of the IEGM can be done either with unipolar or bipolar stimulation and detection.
U.S. Pat. No. 6,473,650 describes an ER detector. The basis for the detection is the idea that the electrode polarisation depends on the stimulation pulse amplitude, while the ER signal does not depend on this amplitude. The sensed signal is sampled and the DC level determined before the delivery of the pulse is subtracted from each sample.
U.S. Pat. No. 5,697,957 describes the suppression of electrode-polarisation components when detecting ER. The sensed cardiac signal is added to either a differentiated or autocorrelated sensed cardiac signal and a difference is formed between the original sensed cardiac signal and the autocorrelated or differentiated signal, thereby extracting an ER component from the sensed cardiac signal.
U.S. Patent Application Publication No. 2003/0083711 describes ER detection by comparing the detected signal with template wave forms. The ER is classified as representing a type of capture if the ER waveform highly correlates with a certain template waveform.
Also U.S. Patent Application Publication No. 2003/0050671 describes ER detection that involves the correlation between a sensed signal and a template waveform. The document describes in particular a method of identifying fusion beats.